C＃继续在一行ForEach

Question

Why I can't do that ? How to manipulate code ?

System.IO.Directory.GetFiles(@"..\DATA\data", "*.pdf").ToList().ForEach(item => item.Contains("TOPRINT") ? continue : System.IO.File.Delete(item));

Answer 1

Linq is designed for operating on sets of data - therefore, all queries must return a set.

You'll have to do something like this, to get a set of the required data:

var filesToDelete = System.IO.Directory
    .GetFiles(@"..\DATA\data", "*.pdf")
    .Where(item => !item.Contains("TOPRINT"));

Then operate on the data-set returned:

foreach(var file in filesToDelete)
    System.IO.File.Delete(file);

Answer 2

Please, don't try doing what you tried doing, and don't do this:

System.IO.Directory.GetFiles(@"..\DATA\data", "*.pdf")
    .Where(p => p.Contains("TOPRINT"))
    .All(p => { System.IO.File.Delete(p); return true; });

LINQ expressions should be side-effect free. Deleting a file is surely a side effect. The ForEach method of List<T> is conceptually wrong (child of a pre-LINQ era). Transforming an array in a List<T> just to use the ForEach is doubly conceptually wrong. What I wrote at least is only "singly" conceptually wrong. (but it WILL work).

Ah... and do what Dave Bish suggested! foreach is the right place for side-effects!

Just remember that his filesToDelete will be parsed lazily, so if you enumerate it twice in two foreach , the Contains check will be done twice!

This considered, there are many powerful static methods in the Array class. It is a pity no one uses them.

var filesToDelete = System.IO.Directory.GetFiles(@"..\DATA\data", "*.pdf");
filesToDelete = Array.FindAll(filesToDelete, p => p.Contains("TOPRINT"));

Array.ForEach(filesToDelete, System.IO.File.Delete); // or
// Array.ForEach(filesToDelete, p => System.IO.File.Delete(p));            
// or, better,  foreach (var file in filesToDelete) { System.IO.File.Delete(file); }

Answer 3

System.IO.Directory.GetFiles(
    @"..\DATA\data", "*.pdf").
    ToList().
    ForEach(
        item => 
        {
            if (item.Contains("TOPRINT"))
                System.IO.File.Delete(item);
        }
    );

or

(from file in System.IO.Directory.GetFiles(@"..\DATA\data", "*.pdf")
 where file.Contains("TOPRINT")
 select file).ToList().ForEach(item => System.IO.File.Delete(item));

Answer 4

I would add where before ForEach

System.IO.Directory.GetFiles(@"..\DATA\data", "*.pdf")
      .Where(item => !item.Contains("TOPRINT")).ToList()
      .ForEach(f=> System.IO.File.Delete(f));